Many direct view and projection display systems are based on liquid crystal display (LCD) technology that require light of a single polarization state. Since most light sources produce light with mixed polarization states, such display systems typically use half of the provided light and discard the other half. In order to enhance the brightness of a display system, many polarization conversion systems have been developed to convert the polarization state of the discarded light to a polarization state usable by the display system.
Known polarization conversion systems typically split a light beam into two sub-beams according to their polarization states, change the polarization state of one sub-beam to a usable polarization state using a wave plate, and then recombine both sub-beams, sending them through the display system.
The more advanced systems use an array of polarization beam splitters (PBSs) coupled either with a fly's eye lens system or an integrating rod. Recent polarization conversion systems use either a limited number of PBSs or a single reflective polarizer coupled to an integrating rod, thus, providing more compactness and lower cost than the ones that use an array of PBSs. Examples of such polarization conversion systems are shown in FIGS. 1A-1E.
FIG. 1A shows a perspective view of a prior art polarization conversion system 25 consisting of an apertured reflective plate 21, a light rod or tunnel 22, a quarter wave plate 23 and a reflective polarizer 24. Input light 19 is focused into the aperture 20 of the reflective plate 21 and travels toward the reflective polarizer 24, which reflects light with one polarization state (e.g., s state) and passes light with an orthogonal polarization state (e.g., p state). The reflected light (e.g., s state) passes through the quarter wave plate 23 and continues toward the apertured reflective plate 21. Some of this light passes through aperture 20 toward the light source and the rest is reflected toward the reflective polarizer 24 by the reflective plate 21. Since the polarization state of this light is converted into the orthogonal state (e.g., p state) after passing through the quarter wave plate 23 for the second time, this light passes through the reflective polarizer 24 when it reaches it the second time. This effectively converts unpolarized input light into polarized output light without discarding a large portion of the input light energy, and thus, improves the intensity of the polarized output light.
FIGS. 1B and 1C show two prior art polarization conversion systems 35 and 45 similar to that of FIG. 1A, except for the replacement of the reflective polarizer 24 by two polarization beam splitters 30 and 31 (FIG. 1B) and a mirror 40 with a single polarization beam splitter 41 (FIG. 1C). Polarization conversion systems of FIGS. 1A-1C have been described in Published European Patent Application No. 1,315,022 A1, to Drazic, Hall and O'Donnell, which is hereby incorporated by reference.
FIGS. 1D-1F use polarization beam splitters (PBSs) and mirrors as a replacement for the apertured reflective plate 21 of FIGS. 1A-1C, thus, providing a higher efficiency.
FIG. 1D shows a perspective view of a prior art polarization conversion system 65, which consists of two polarization beam splitters 60a and 60b, a rhomb 62, a half wave plate 63 and a light pipe 64. Input light 61 is focused into the first PBS cube 60a as shown in FIG. 1D. Light with one polarization state (e.g., p state) is transmitted to the light pipe 64 and light with orthogonal polarization state (e.g., s state) is reflected toward the second PBS cube 60b. At the surface of the second PBS cube 60b, light with an orthogonal polarization state (e.g., s state) is reflected toward the half wave plate 63 where its polarization state is converted into the orthogonal state (e.g., p state) and enters the light pipe 64. Such a system 65 has been commercialized by OCLI, Inc., A JDS Uniphase Company of Santa Rosa, Calif.
FIG. 1E shows a perspective view of a prior art polarization conversion system 80, which consists of a polarization beam splitter cube 73, a prism reflector 71, a half wave plate 74, a spacer 75 and a light pipe 76. Input light 72 is coupled into the PBS cube 73 either directly as shown in FIG. 1E or through other arrangements such as a tapered light pipe. Light with one polarization state (e.g., p state) is transmitted to the light pipe 76 through the spacer 75 and light with the orthogonal polarization state (e.g., s state) is reflected toward a prism reflector 71. At the surface of the prism reflector 71, light with the orthogonal polarization state (e.g., s state) is reflected toward the half wave plate 74, where its polarization state is converted into the other state (e.g., p state) and enters the light pipe 76.
FIG. 1F shows a perspective view of a prior art polarization conversion system 100, which consists of a polarization beam splitter cube 93, a prism reflector 91, a quarter wave plate with a reflector 92 and a light pipe 94. Input light 95 is coupled into the PBS cube 93 as shown in FIG. 1F or delivered via a tapered light pipe (not shown). Light with one polarization state (e.g., p state) is transmitted to the prism reflector 91, which in turn reflects it toward the light pipe 94. Light with the orthogonal polarization state (e.g., s state) is reflected toward the quarter wave plate 92 where it enters and exits the quarter wave plate 92 toward the light pipe 94 with the opposite polarization state (e.g., p state). The systems 80,100 are further described in U.S. Pat. No. 6,587,269 B2, to Kenneth K. Li, which is hereby incorporated by reference.
It is important that polarization conversion systems operate with minimal light loss, are physically compact, and relatively inexpensive. Although known polarization converters are useful in some applications, there is a need for improved polarization conversion systems that are more compact, light weight, efficient and cost-effective.